1. Field of the Invention
The present invention relates to an apparatus for detecting timing for starting the combustion in an internal combustion engine based on a microwave signal and a luminous signal.
2. Description of the Prior Art
Improved exhaust emission and fuel economy for internal combustion engines strongly require accurate detection of conditions in the combustion chambers, the top dead center position, and fuel injection and ignition timing with respect to the top dead center while the engine is in operation. Such accurate detection of condition and timing is also useful for analyzing combustion to achieve the better emission and mileage. Reflecting the recent trend for more delicate combustion control, there is a strong demand on production lines for an apparatus capable of accurately and simply determining whether manufactured internal combustion engines inject fuel at desired timing or angle with respect to the top dead center and ignite the air-fuel mixture.
More and more diesel engines are used on passenger cars because of their better fuel economy. Since diesel engines found widespread use much later than gasoline engines and fail to provide ready information for timing detection, more need has arisen particularly recently for a timing detecting apparatus for use with diesel engines.
Pollutants HC and NOx in the exhaust gas discharged from diesel engines vary greatly in amount when injection timing of the fuel injection pump deviates from a setting even by 1 degree in terms of crank angle. Therefore, it has heretofore been difficult to control the accuracy of fuel injection timing in mass-produced diesel engines. It has been necessary to determine statically two relative positions, that is, a standard crank angle position and an injection pump attachment position, which serve as a reference for injection timing, and the accuracy of determined injection timing has been on the order of several degrees.
With mass-produced diesel engines, consequently, it has to take into account that the injection timing of the fuel injection pump is variable in .+-.3.degree. of a setting. The diesel engines should be equipped with an exhaust emission control device such as EGR (Exhaust Gas Recirculation) to meet the emission control requirements even when the injection timing happens to differ from the setting. Therefore, the overall engine system is complex and the fuel economy is poor.
To avoid such problems, an adjustment system has been put to use which is capable of dynamic timing adjustment with the accuracy of .+-.0.5.degree., by utilizing microwave resonance as described in U.S. Pat. No. 3,589,177.
There is an adjustment system utilizing the phenomenon of microwave resonance, which detects the top dead center and represents fuel injection timing with a luminous signal generated upon fuel combustion in a pre-chamber. The difference in crank angle between the detection of the top dead center and the luminous signal is detected and adjusted so as to be within .+-.0.5.degree. of the setting by automatically changing the injection pump attachment position with a stepper motor.
The adjustment system comprises a microwave unit for generating and detecting a microwave, a probe for introducing the microwave into the engine and detecting light emitted from the pre-chamber as a luminous signal, an encoder for picking up a crank angle signal, an electronic circuit for determining a top dead center by processing the microwave signal based on the crank angle signal from the encoder, determining the crank angle difference between the luminous signal from the pre-chamber and the detection of the top dead center, and delivering a command signal to a stepper motor so as to bring the crank angle difference into conformity with a setting, and the stepper motor for changing an injection pump attachment position.
The waveform and peak value of the luminous signal generated from the pre-chamber vary with the load on and the speed of the engine, and the degree of variation in the waveform and the peak value is different in each cycle. Under low loads, in particular, the waveform and the peak value tend to vary by 50% in each cycle. It has been customary practice to detect the luminous signal simply in terms of the time or angle at which the luminous signal exceeds a preset level, the detected time or angle being indicative of a rise time for the luminous signal. This rise time varies widely depending on operating conditions, resulting in a poor accuracy of the system. To avoid such a difficulty, it has been necessary to keep the engine load at a low level, 1,300 RPM for example, for thereby reducing the variation of the luminous signal from the pre-chamber in each cycle. An additional device is therefore required for maintaining the engine under such a load condition. Since the engine operating range that can be detected is limited, the sensor is less effective in operation.
Since the microwave resonance is caused dependent on the piston position, the microwave signal indicative of the top dead center is available as a time signal while the engine is in rotation. The amounts of harmful components NOx and HC in the exhaust gas are determined by the crank angle position of the fuel injection pump, and hence the top dead center should be found as a crank angle. Therefore, an angle signal has also been employed. High accuracy required of such an angle signal has precluded the use of an ordinary angle sensor used on engines, such as a sensor composed of an electromagnetic pickup simply attached to a ring gear, as the sensor fails to provide a sufficient accuracy (.+-.0.1.degree.). Conventionally, an angle sensor and an encoder which are of higher accuracy should be attached through a shaft driver and the like in each adjustment process.
It has been troublesome to attach the angle sensor and encoder each time an adjustment is to be made. The time required for measurement has been increased. An engine attachment system should newly be added. In addition, special consideration should be given to accuracy and reliability since the accuracy is dependent on the angle sensor and encoder. The sensor accuracy cannot be diagnosed by the output from the sensor itself. This has necessitated higher accuracy and reliability of the encoder and also higher reliability and reconstructibility of the engine attachment system.